1. Field of the Invention
The present invention relates generally to nuclear reactors and, more particularly, is concerned with a unique design concept for burnable absorber rods which provides rods of an overall standardized length while simulating conventional custom-designed rods which have a variety of reduced lengths.
2. Description of the Prior Art
In a typical nuclear reactor, the reactor core includes a large number of fuel assemblies each of which is composed of top and bottom nozzles with a plurality of elongated transversely spaced guide thimbles extending between the nozzles and a plurality of transverse grids axially spaced along the guide thimbles. Also, each fuel assembly is composed of a plurality of elongated fuel elements or rods transversely spaced apart from one another and from the guide thimbles and supported by the grids between the top and bottom nozzles. The fuel rods each contain fissile material and are grouped together in an array which is organized so as to provide a neutron flux in the core sufficient to support a high rate of nuclear fission and thus the release of a large amount of energy in the form of heat. A liquid coolant is pumped upwardly through the core in order to extract some of the heat generated in the core for the production of useful work.
Since the rate of heat generation in the reactor core is proportional to the nuclear fission rate, and this, in turn, is determined by the neutron flux in the core, control of heat generation at reactor start-up, during its operation and at shutdown is achieved by varying the neutron flux. Generally, this is done by absorbing excess neutrons using control rods which contain neutron absorbing material. The guide thimbles, in addition to being structural elements of the fuel assembly, also provide channels for insertion of the neutron absorber control rods within the reactor core. The level of neutron flux and thus the heat output of the core is normally regulated by the movement of the control rods into and from the guide thimbles.
Also, it is conventional practice to design an excessive amount of neutron flux into the reactor core at start-up so that as the flux is depleted over the life of the core there will still be sufficient reactivity to sustain core operation over a long period of time. In view of this practice, in some reactor applications burnable absorber or poison rods are inserted within the guide thimbles of some fuel assemblies to assist the control rods in the guide thimbles of other fuel assemblies in maintaining the neutron flux or reactivity of the reactor core relatively constant over its lifetime. The burnable poison rods, like the control rods, contain neutron absorber material. They differ from the control rods mainly in that they are maintained in stationary positions within the guide thimbles during their period of use in the core. The overall advantages to be gained in using burnable poison rods at stationary positions in a nuclear reactor core are described in U.S. patents to Rose (U.S. Pat. No. 3,361,857) and to Wood (U.S. Pat. No. 3,510,398).
U.S. Pat. Nos. 4,169,759 and 4,169,760 to Bevilacqua describe the use of a first group of full length control rods insertable into a nuclear reactor core for normal control of reactor power and of a second group of part length control rods insertable into the core for control of power oscillations. The part length control rod has first and second ends with a first neutron absorbing material at its first end, a second neutron absorbing material at its second end spaced from the first neutron absorbing material by a distance less than the length of the core, and a third intermediate portion connecting the first and second neutron absorbing material, the intermediate material being substantially non-neutron absorbing. The first neutron absorbing material is normally positioned outside of the reactor core where it has little or no effect on the neutron flux of the reactor core, while the second neutron absorbing material is normally positioned in the central region of the core for control of power oscillation. Upon the requirement for a rapid reactor shutdown, the part length control rod is scrammed or inserted into the core so that both first and second ends of the control rod are present in the core.
The current trend in reactor core power distribution control is toward the use of burnable absorber rods having reduced lengths for the maximization of peaking factor margin. This approach involves the provision of custom-designed lengths and placements of the reduced length burnable absorber or poison rods. However, such approach presents some potentially significant manufacturing and handling difficulties. First, it precludes the manufacturing facility from being able to build rods from a standard inventory and can require a significant additional non-standard inventory (to cover manufacturing yield problems) that is unusable after completion of the reduced lenght rods for one particular reload region of the core. Second, handling of reduced length burnable absorber rod clusters when loading or unloading the reactor core entails considerable difficulty. Because of the design of the burnable absorber rod cluster handling tool, it is a very difficult task to be able to deal with clusters of a length different from that originally assumed in the design of the tool. It is doubly difficult because the cluster is sufficiently delicate that handling it improperly can result in damage, which could result in a delay in restarting the reactor.
Consequently, a need exists for a different approach to burnable absorber rod design which will retain the flexibility of custom-designed reduced length burnable absorber rods in terms of maximizing reactor core power distribution control capability while, if not eliminating, at least reducing the above-described difficulties encountered in the manufacturing and handling of these custom-designed rods.